1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device using visible-range light as well as infrared-range light as light sources.
2. Discussion of the Related Art
With the progress of information-dependent society, the demand for various display devices has increased. To satisfy such demand, efforts have recently been made to develop flat panel display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electro luminescent displays (ELDs), and vacuum fluorescent displays (VFD). Some types of such flat panel displays are being practically applied to various appliances for display purposes.
Among these, LCDs are currently the most widely used as a substitute for cathode ray tubes (CRTs) in association with mobile image display devices because LCDs have advantages of excellent image quality, lightness, slimness, and low power consumption. Various applications of LCDs are being developed in association with not only mobile image display devices such as monitors of notebook computers, but also monitors of TVs to receive and display broadcast signals, and monitors of computers.
FIG. 1 is a cross-sectional view showing a conventional liquid crystal display device.
Referring to FIG. 1, a conventional liquid crystal display device includes a liquid crystal panel 10 including upper and lower substrates 10a and 10c opposite to each other and a liquid crystal layer 10b interposed between the upper and lower substrates 10a and 10c and a backlight unit 20 disposed over the bottom surface of the liquid crystal panel 10.
Since the liquid crystal display device as described above includes liquid crystal molecules having a thin and long structure and aligned in a predetermined orientation, the orientation of the liquid crystal molecules may be controlled by applying an electric field to the liquid crystal layer 10b. That is, liquid crystal molecules are arranged according to the electric field applied to the liquid crystal layer 10b to pass or block light received from the backlight unit 20, thereby displaying an image or text.
In particular, polarizing films 12 are attached to external surfaces of the upper and lower substrates 10a and 10c. In this regard, a polarization axis of the polarizing film 12 attached to the external surface of the upper substrate 10a is perpendicular to a polarization axis of the polarizing film 12 attached to the external surface of the lower substrate 10c. 
Particularly, light emitted from the backlight unit 20 is polarized in a predetermined direction while passing through the polarizing film 12 disposed over the bottom surface of the liquid crystal panel 10. The light polarized in the predetermined direction is incident upon the liquid crystal panel 10. If the liquid crystal panel 10 is in a normally black mode in which black is displayed when a voltage is not applied, light polarized in a predetermined direction maintains the polarized direction while passing through the liquid crystal panel 10 to which voltage is not applied. The light cannot pass the polarizing film 12 of the top surface of the liquid crystal panel 10 and is absorbed by the polarizing film 12 to realize black. On the other hand, when a voltage is applied to the liquid crystal panel 10, light polarized in a predetermined direction is polarized in the opposite direction while passing through the liquid crystal panel 10 and passes through the polarizing film 12 of the top surface of the liquid crystal panel 10 to display an image.
In this regard, the polarizing film 12 is an absorption-type polarizing film prepared by dying poly vinyl alcohol (PVA) with iodine and controls polarization characteristics. Of light incident on the polarizing film 12 as described above, light vibrating in a direction in which iodide ions such as I3− and I5− are arranged is absorbed by the polarizing film 12, and light vibrating in other directions passes therethrough. However, the polarizing film including iodide ions may only polarize visible-range light.
FIG. 2 is a graph illustrating absorbance of iodide ions.
As shown in FIG. 2, I5− ions absorb red light with a wavelength in the range of 500 nm to 700 nm, and I3− ions absorb blue light with a wavelength in the range of 400 nm to 500 nm. Thus, a polarizing film including I5− ions and I3− ions has a low absorbance level at a wavelength less than 400 nm and greater than 700 nm, so as not to perform polarization of infrared-range light with a wavelength over 780 nm.